The invention relates to bearings. More specifically, the invention relates to a foil thrust bearing including a follower spring.
Foil thrust bearings are very attractive for high speed turbomachines because they do not require oil-based lubrication and the maintenance demands of an oil-based lubrication system. Instead of using oil as a bearing lubricant, a foil thrust bearing uses a process fluid, such as air, methane, water, gaseous or liquid hydrogen, nitrogen or oxygen.
A turbomachine typically includes a rotor that is supported in an axial direction by at least one thrust bearing. A foil thrust bearing for the turbomachine is disposed between a stationary housing member and a rotational thrust runner. The foil thrust bearing includes a foil assembly including a backing plate and a circular array of circumferentially-extending foils carried by the backing plate. Additionally, the foil thrust bearing includes a spring assembly for providing compliant pressure axially to each overlying foil of the foil assembly. When the runner is rotated relative to the housing, a fluid film is generated between the foil assembly and the runner. At high rotational speeds, a pressure build-up within the fluid film supports the runner, while maintaining a separation between the runner and the foil assembly.
The rotor might have a certain amount of travel or free play in the axial direction. A large axial travel of the rotor might reduce or remove the compliant pressure placed on the foils by the spring assembly. Reducing or removing the compliant pressure would degrade the performance of the foil thrust bearing.
In such situations, the foil thrust bearing might be provided with a follower spring underlying the spring assembly. The follower spring pre-loads the spring assembly to ensure that compliant pressure is maintained on the foils throughout the axial travel of the rotor.
However, the follower spring adds overall size to the foil thrust bearing, increasing its thickness and radial cross-section. Increasing the size of the foil thrust bearing can be undesirable.
Moreover, there might be packaging constraints on the foil thrust bearing. Consequently, the follower spring might not size properly, particularly for foil thrust bearings that require small physical sizes and bearings that have larger sizes but require smaller radial widths. Additionally, the follower spring for such bearings might not be able to provide a proper axial preload. Performance of the foil thrust bearing would suffer.